This photograph of me with my sons was taken at our home in Mexico City in 2010. It's from my series Kinderwunsch; in my first language, German, this means "the desire to have children". I began working on the series seven years ago, following the birth of my first boy, Martin. It had taken me a long time to get pregnant. Then, after his arrival, I began fertility treatment again in the hope of having another child. I decided to document the whole process – right through to the birth of Lucio and beyond.

As the boys got older, I wanted to actively involve them in my work. I asked them to suggest games we could play. We bathed in milk; they covered me in Play-Doh. On this occasion, they wanted to draw on me. I set up the lights and camera in the playroom, got out the pens, then lay down on the carpet. Lucio was two and still breastfeeding, so he took my breast in his mouth as he drew. He wasn't feeding – it just felt natural for him. I then took lots of pictures using remote control.

When the boys had finished, I took a self-portrait before going off to have a bath. I see their scribbles on my body as a symbol of how motherhood has changed me. Although I am much more aware of my fragility these days, I find I am more able to live for the present, and share those fleeting moments in childhood when important things happen. I have adapted the way I work, too: I switched to colour for the series in order to capture our skin tones. And I worked in digital because kids just move too quickly for film.

The nudity in my work often provokes strong reactions. The weirdest responses to Kinderwunsch have come from men: male editors have told me they can't publish the series because it's "too real". I know there is a fine line between perceiving the body as a sensual object and as a non-sensual one – but that is all tied up with being a mother. This image captures the physical bond between myself and my sons. I am their canvas: they play with me and change me.